A radiocommunication system SY comprising a first broadband radiocommunication system SYBB and a second narrowband radiocommunication system SYNB which are deployed on the same radioelectric transmission sites in a determined geographical zone is known. The operator of these sites can thus offer over this same zone at one and the same time narrowband services and broadband services. According to the prior art, these two systems operate in separate frequency bands to avoid mutual interference.
With reference to FIG. 1, the radiocommunication system SY comprises a plurality of sites, called cells C1 to CC. For a better understanding of FIG. 1, only 4 cells C1, C2, C3 and Cc are detailed. Each cell Cc, with 1≦c≦C, comprises first and second base stations, respectively BSBB,c, BSNB,c and mobile stations MS1 to MSK which communicate with the base stations through the radio resources shared in the respective frequency bands ΔFsyBB for broadband communications and ΔFsyNB for narrowband communications. More particularly, each cell Cc comprises a first base station BSBB,c, called a broadband base station BSBB,c in the subsequent description, able to communicate radioelectrically with mobile stations in a broadband radiocommunication network of the first radiocommunication system SYBB. Each cell Cc also comprises a second base station BSNB, called a narrowband base station BSNB,c in the subsequent description, able to communicate radioelectrically with mobile stations in a narrowband radiocommunication network of the second radiocommunication system SYNB. The mobile stations present in a cell and operating according to a single one of the two modes of communication, broadband or narrowband, register respectively with one of the two base stations BSBB,c or BSNB,c according to their mode of operation. Mobile stations operating according to both modes of communication can register with one of the two base stations by choice or with both base stations.
For radiocommunication systems SYBB and SYNB of FDD (Frequency Division Duplex) type, the respective predetermined frequency bands ΔFsyBB and ΔFsyNB each comprise a first frequency band ΔFsyBBe, respectively ΔFsyBBe, for the emission of communications from the base stations BSBB,c or BSNB,c to the mobile stations, supplemented with a second frequency band of the same width ΔFsyBBr, respectively ΔFsyBBr called the duplex band, for the receptions of communications originating from the mobile stations by the base stations BSBB,c or BSNB,c. The first frequency band ΔFsyBBe, respectively ΔFsyBBe and the second associated frequency band ΔFsyBBr, respectively ΔFsyNBr are shifted by one and the same duplex gap ΔFD.
The broadband radiocommunication system SYBB is for example of the WIMAX (“Worldwide Interoperability for Microwave Access”) type based on an Air interface according to the IEEE 802.16 standard, more particularly according to the 802.16m standard, or for example of the LTE (Long Term Evolution) standard which employs wide frequency bands ΔFsyBBe and ΔFsyBBr each typically greater than a Mega-Hertz, for example 1.25 MHz, 1.4 MHz, 3 MHz, 5 MHz, 10 MHz or 20 MHz.
As shown in FIG. 2A, in the broadband radiocommunication system SYBB, each predetermined frequency band ΔFsyBBe and ΔFsyBBr is divided into J frequency blocks respectively BFe1 to BFeJ and BFr1 to BFrJ, each of bandwidth ΔBF, typically of a few hundred Kilo-Hertz, for example ΔBF=180 kHz in the case of a system according to the LTE standard. Each block BFej, BFrj, with 1≦j≦J, comprises N consecutive and regularly distributed carrier frequencies Fj,1 . . . Fj,n, . . . Fj,N of channel width ΔF=ΔFsye/(J×N), with 1≦n≦N. For example, in the case of the LTE standard, N is equal to 12 and the interval ΔF between two consecutive sub-carriers is equal to 15 kHz, so that ΔBF=N×δF=12×15 kHz=180 kHz.
Radio resources are allocated to a base station BSBB,c for a high data throughput transmission to (or from) a mobile station operating at least in broadband mode. FIG. 2B is an illustration of the radio resources shared by the broadband base stations BSBB in a downlink communication channel in the frequency band ΔFsyBBe during a time frame TP, and are similar in the uplink communication channel (not represented). A communication channel, downlink or uplink, of the LTE broadband system corresponds to the set of resources in the frequency band ΔFsyBBe (or ΔFsyBBr) during a time frame TP. The radio resources are blocks of resources, each BRj,tp defined on a frequency block BFej (or BFrj depending on the direction of the channel) during a specific time window tp, called a time pitch, consisting of several symbol times within the meaning of OFDM modulation. A communication channel comprises common sub-channels CNC for synchronization and broadcasting of the system information between the broadband base stations, and transport sub-channels for exchanges of data and of signaling between the base stations and the mobile terminals. The common sub-channels CNC correspond to a set of resource blocks extending over a few contiguous frequency blocks (six in the case of LTE) for a few symbol times and are repeated in part in the time frame TP. The other blocks of resources correspond to the transport sub-channels and are shared between the C base stations BSBB,1 to BSBB,c of the radiocommunication system SYBB according to a known method for allocating resources, such as frequency reuse according to a specific factor for example a factor of 3 or a factor of 1, or such as fractional frequency reuse. With reference to FIG. 2B, on the frequency plan, several frequency blocks, for example blocks BFej to BFej+5, comprise few resource blocks intended for the sub-channels CNC and resource blocks intended for the transport sub-channels. The other frequency blocks comprise resource blocks intended solely for the transport channels, for example the frequency block BFe1 with reference to FIG. 2B.
The narrowband radiocommunication system SYNB is for example a TETRA (“TErrestrial Trunked RAdio”) or TETRAPOL system whose channel width δf is of the order of a few Kilo-Hertz for example 10 kHz, 12.5 kHz or 25 kHz, this width δf also being the frequency pitch separating two carrier frequencies. With reference to FIG. 3A, the uplink and/or downlink communication frequency channel of the narrowband system between a narrowband base station and a mobile terminal corresponds to a carrier frequency fec,p or frc,p (represented fe/rc,p in FIG. 3A) of channel width δf. The useful bandwidth δb of the filtered frequency signal is less than the width of the channel δf. For example, for a channel width δf of 10 KHz the bandwidth δb will be for example 8 KHz.
With reference to FIG. 3B, in the narrowband radiocommunication system SYNB of FDD type, the usual distribution of the frequency plan is such that to each cell Cc are allocated two groups of P carrier frequencies fec,1 . . . fec,p, . . . fec,P and frc,1 . . . frc,p, . . . frc,P of channel width δf, which are respectively distributed over the frequency bands ΔFsyNBe and ΔFsyNBr. For each frequency band ΔFsyNBe and ΔFsyNBr the distribution of the narrowband carrier frequencies allocated to one and the same base station, in one and the same cell Cc, complies with certain constraints between said frequencies.
A first constraint relating to the use of conventional coupling systems, more particularly coupling systems using cavities, for transmitting messages from the base station BSNB,c to the mobile terminals present in the cell, requires compliance with a first minimum frequency interval Δfe between the carrier frequencies used in one and the same cell, for example Δfe=150 kHz.
A second constraint makes it possible to avoid disturbances related to the use of too close frequency channels to transmit messages to the base station BSNB,c, at one and the same time, by mobile terminals close to the base station BSNB,c and mobile terminals far removed from the base station BSNB,c. This constraint imposes compliance with a second minimum frequency interval AΔfr between said carrier frequencies of one and the same cell, for example Δfr=20 kHz, and which may be less than the first pitch Δfe.
As the frequency channels for uplink communication in the direction from mobiles to base station correspond, to within the duplex gap, to the frequency channels for downlink communication from the base station to the mobile stations, the minimum gaps between channels related to the constraints of the base station will lie identically, to within a frequency translation, in the other frequency sub-band corresponding to the uplink communications from the mobile stations to the base station.
Cells which are geographically sufficiently far apart can have identical carrier frequencies fec,p, frc,p or groups or parts of groups of identical carrier frequencies. The mutual interference of these cells in one and the same frequency channel is very low, the carrier-to-interference ratio determined in each of the cells as a function of the other cell being less than a specific threshold.
Standard allocations, such as these, of frequency blocks and of carrier frequencies are effective when they are applied respectively to a first and a second radiocommunication system, SYBB and SYNB, located in distinct geographical zones, and/or working on distinct frequency bands ΔFsyBB, ΔFsyNB. If the communication systems SYBB and SYNB, according to the invention, are located in one and the same geographical zone and share the same emission and reception frequency bands ΔFsye and ΔFsyr the allocations of carrier frequencies on the one hand, and of frequency blocks, more particularly the transport channels, on the other hand, will produce mutual interference having a very negative effect on the service quality of said communication systems.
Indeed, according to a typical exemplary configuration, the carrier frequencies of the narrowband system SYNB have a channel width Δf of 10 KHz and the first frequency interval Δfe between two carrier frequencies of one and the same cell Cc is 150 KHz. By assuming that each frequency block BFej, BFrj of the broadband system SYBB has a bandwidth ΔBFj of 180 KHZ for the LTE systems, several frequency blocks, indeed all the frequency blocks potentially used by the broadband base station BSBB,c of the cell Cc can each contain at least one carrier frequency of the narrowband base station BSNB,c belonging to the same cell Cc and be interfered with by these carrier frequencies.
It is possible to limit this drawback by avoiding allocating a frequency block to a given cell, stated otherwise by neutralizing the block, when its allocation would be liable to create interference at the carrier frequencies of the narrowband system that are allocated in the same given cell or in cells sufficiently near to this given cell to undergo interference. Thus, these interfered frequency blocks become unusable by application of a strategy for sharing the radiocommunication system SY prohibiting the allocation of a frequency block BFej, BFrj to a broadband base station BSBB if it is interfered with by a carrier frequency of a base station BSNB located in the same cell or in a geographically close cell. The application of such a strategy mutually ensures the protection of the frequency blocks of the broadband system. Nonetheless, the number of neutralized frequency blocks may, in the configuration represented hereinabove, very severely reduce the capacity of the broadband communication system.
To alleviate this drawback, it is known to use multi-carrier frequency transmitters in the narrowband base stations of the narrowband communication system SYNB. Such a transmitter groups together the carrier frequencies allocated to one and the same base station BSNB,c into a group of carrier frequencies distributed consecutively over a not very extended frequency band with a small frequency interval Δlfe between each carrier frequency, for example Δfe goes from 150 KHz to 20 KHz. The carrier frequency group allocated to the base station BSNB,c of the cell Cc thus has a frequency bandwidth, for example of 140 KHz in the case of a group of 8 frequencies, that is less than the bandwidth of a frequency block, which in the previous example is 180 KHz. Depending on its position with respect to the frequency blocks, the group of carrier frequencies interferes with only one or two frequency blocks at most. The other frequency blocks that are not interfered with by this group of frequencies may potentially be allocated to the broadband base station BSBB,c belonging to the cell Cc. However, the carrier frequency groups allocated respectively to the narrowband base stations lying respectively in the cells adjacent to the cell Cc, may nonetheless be distributed over the whole of the frequency band of the radiocommunication system SY and thus interfere with several frequency blocks, indeed all the frequency blocks distributed over the frequency bands ΔFsye and ΔFsyr, rendering them unusable for the broadband base station BSBB,c of the cell Cc.